The objective of this grant is to investigate, on molecular terms, the mechanism of induction of the acrosome reaction. We have found that when rabbit sperm are pretreated with media of high ionic strength (380 mOsM), previously shown to facilitate removal of sperm bound seminal plasma components, and subsequently treated with follicular fluid, the acrosome reaction is completed rapidly. Treatment of the sperm with follicular fluid alone yields a greatly decreased rate of acrosome reaction completion and treatment with the high ionic strength medium alone causes no visible alteration to the sperm. These results suggest that removal of the sperm bound seminal plasma components destabilizes the acrosome and prepares it to undergo the acrosome reaction. Results obtained indicate that this destabilization is virtually completed during five minutes of preincubation of the sperm in high ionic strength media. Direct comparison with epididymal sperm acrosomes indicates they are apparently in the same stabilized condition as ejaculated sperm. The effect of the pretreatment by high ionic strength media can be partially mimicked by pretreatment of sperm with gamma or B-amylase or neuraminidase but not with B-glucuronidase, lipase, pronase, or trypsin. Comparison of the ability of bovine follicular fluid, rabbit follicular fluid or rabbit serum to induce the rabbit acrosome reaction shows that bovine follicular fluid is 3 to 4 times more effective than rabbit follicular fluid and that rabbit serum is totally ineffective in producing the acrosome reaction. Further characterization of the acrosome reaction inducing activity of follicular fluid is presently underway. BIBLIOGRAPHIC REFERENCES: Capacitation of Rabbit Spermatozoa in vitro. B.G. Brackett and G. Oliphant. Biol. Reprod. 12, 260 (1975).